madmaya:

Yet the human body is'nt efficient enough to totally assimilate what is ingested or maybe what is excreted does have a definite purpose.... for ex, I think in China long back, "night soil" was a valuable fertilizer?

Not so long back. As recently as the 80's, I was in a chinese village where the effluence is hauled away by the "night gentleman". All the crap is taken to a boat, and eventually it is sold back to the farmers as fertilizer.